Organophosphorus pesticides (OPs), a large and chemically diverse class, are the most commonly used and economically important insecticides worldwide, accounting for approximately 40% of recently used insecticides in the U.S. While legal OP concentrations are not acutely toxic to humans, studies suggest that chronic prenatal and infant exposures can lead to life-long neurological damage and behavioral disorders. Acute OP poisoning due to inhibition of acetylcholinesterase (AChE) is well-understood. But, despite decades of OP research, it remains debated whether and how subacute OP exposure at regulated levels that do not significantly inhibit AChE causes morphological and cognitive defects in the developing human brain. Much of this controversy is because connections between target molecules/pathways and adverse health outcomes are largely unknown. Most of what we know about OP developmental neurotoxicity (DNT) comes from studies on the most abundant OP, Chlorpyrifos (CPF). But because OPs are chemically diverse with different pharmacokinetics, and possibly pharmacodynamics, it is extremely difficult to predict how different OPs act from CPF alone. Additionally, since humans often encounter several OPs simultaneously, due to their ubiquity and frequent use in mixtures, it is imperative to understand the toxicity mechanisms of different OPs individually and in combinations to unravel possible non-additive toxic effects and to accurately predict toxicity of real-life combinatorial exposure to humans. However, systematic studies of multiple OPs to reveal these connections are impossible in traditional mammalian models, which are inherently expensive and low-throughput. We hypothesize that phenotypic differences of OP DNT result from interactions with different molecular targets, therefore causing exposure to OP mixtures to have largely non-additive neurotoxicity. We will test this hypothesis by executing a comparative screen of OP neurotoxicity using the asexual freshwater planarian Dugesia japonica, an innovative high-throughput invertebrate system pioneered by the PI. Planarians have a simple, tractable brain, which can reform de novo in 2 weeks post-amputation, allowing us to induce development ?at will? by amputation and screen adult and developing/regenerating animals with the same assays to delineate development-specific effects. This project aims to: (1) Connect the molecular targets underlying OP DNT with organismal phenotypes by comparing endpoints affected by individual OPs with those affected by chemicals with known targets and (2) Screen binary OP mixtures to reveal possible synergistic and antagonistic effects. Information generated from this project will provide unprecedented insight into target-phenotype connections, possibly transforming our view of alternative OP targets or synergistic effects. Toxicity pathways identified in planarians can then guide targeted mechanistic studies in mammalian systems, speeding up the testing pipeline and strengthening the call for a paradigm shift in prenatal OP exposure guidelines.